Adolescents and young adults are disproportionately affected by HIV/AIDS, with homeless youth estimated to be 6-12 times more likely to become infected with HIV than home-based youth. Homeless youth have high rates of drug use and are embedded in social networks that include regular drug users. Risky sexual behavior, the primary route of infection in this population, is common. Understanding the social context of drug use and risky sex among homeless youth is critical to intervention efforts aimed at reducing the spread of HIV in this growing population. Using probability samples of homeless youth ages 13-23 in Los Angeles County, we will take an innovative and multifaceted approach to addressing the following aims: Aim 1: We will first conduct 72 quantitative social network interviews and 45 qualitative sexual event-level interviews to better understand the social context of sexual behaviors and drug use. Based on findings from Aim 1, we will conduct 400 quantitative structured interviews to address Aims 2-3. Aim 2: To investigate how the social network characteristics of homeless youth are associated with their patterns of drug use;and investigate how social network characteristics and drug use among homeless youth are associated with their propensity toward sexual risk behavior, and the extent to which HIV-related attitudes and beliefs mediate these associations;Aim 3: To investigate how characteristics of specific sexual events and HIV-related attitudes and beliefs are associated with sexual risk behavior;and, for those event-level characteristics found to be associated with sexual risk behavior, to determine how these characteristics are related to homeless youth's social network characteristics, drug use, and HIV-related beliefs and attitudes;Aim 4: Finally, we will conduct 60 event-level qualitative interviews to further explore, within a social context, the patterns of relationships between drug use and sexual risk behaviors identified in Aims 2 and 3.